This invention relates to metering electrical energy in terms of, for example, kilowatt hours in electrical systems and more specifically, to a method and apparatus for metering electrical energy in a single phase, a.c. electrical system having three lines or wires.
Electrical energy in terms of kilowatt hours or kWh has been and continues to be metered with the rotating disc type of meter which is disclosed in, among other sources, the "Electrical Meterman's Handbook", Chapter 7, 7th Edition, published 1965 by Edison Electric Institute. The invention hereinafter disclosed represents a departure from the methodology and apparatus exemplified by the rotating disc-type meter.
In addition, those who are familiar with the instrumentation and metering arts respecting electrical power and energy know of proposed systems, including apparatus employing electronic and solid state devices for measuring power and energy. In such apparatus, the electronic and solid state devices replace the conventional rotating disc. However, these apparatus are primarily directed to polyphase systems or single phase, two-wire systems as opposed to the three-wire system to which the subject invention is applicable.
Thus, in prior art systems, the current flowing through a line and the voltage across a load impedance connected to the line are each detected and signals representing these parameters are coupled to a pulse width amplitude multiplier. The multiplier generates an output signal which is proportional to the product of the detected current and voltage. This signal is then averaged by means of a low pass filter to provide a d.c. voltage having an amplitude which is proportional to the total average power of the electrical system. This d.c. voltage is converted to a pulse train by an analog-to-pulse rate converter, which pulse train has a variable signal repetition rate f which is proportional to the average power in the system, with each output pulse signal representing a constant quantized amount of electrical energy. The output pulse train is then coupled to a display mechanism which may, for example, include a counter or register which performs conventional accumulation storage and display functions. However, none of the prior art measuring systems is designed to measure the power consumed in a single phase, three-wire system wherein a conventional pulse width amplitude multiplier having a single voltage and current derived input is utilized. Accordingly, a multiplier of alternate design is required in order to accommodate the detected input levels on the lines of the three-wire system.
Accordingly, it is an object of this invention to provide a method and apparatus for metering kWh in a single phase, three-wire a.c. electrical system.
It is another object of this invention to provide a method and apparatus for metering active electrical energy in a single phase, three-wire electrical system wherein a multiplier of conventional design, suitable for use in a single phase, two-wire system, may be utilized.
It is another object of this invention to provide a meter for metering active electrical energy wherein the meter includes solid state circuits which may be fabricated in the form of monolithic integrated structures.